The image file names created by WhatsApp looking like this:
Thus, images created by WhatsApp and uploaded to Google Photos can be found by typing this in the Google Photos search bar:
Or even simpler, click on this link:
Please note that this search will also match files that were not created by WhatsApp. Such as files named week-end-at-the-beach.jpg for example. Or any file that has four dashes (-) in its name.